1. Field of Invention
This invention relates to crystal oscillators and more particularly to AT-cut crystal oscillators for electronic timepieces.
2. Prior Art
In conventional crystal oscillators for watches, a tuning fork-type crystal oscillator as shown in FIG. 1 has been broadly used. This type has, however, inferior physical characteristics. Such characteristics include a frequency-temperature characteristic curve of a inverted parabola having a zero temperature coefficient at approximately room temperature as shown in FIG. 2, a total differential in frequency as much as 20 parts per million, and a frequency aging effect of 5 .times. 10.sup.-6 per year. In addition, the fork-shape of the tuning fork oscillator is rather complex to manufacture thereby increasing manufacturing costs. Furthermore, its impact strength and vibration characteristics are not very good because of the complex and weak supporting structure. Accordingly, this type has not been satisfactory as a time standard for either a high quality watch or a low cost watch.
In addition to the tuning fork-type oscillator, shown in FIG. 3 is the configuration of an AT-cut crystal oscillator having a frequency greater than or equal to 4 MHz which is used in electronic communication instruments in general. FIG. 4 shows a distribution of a displacement in vibration mode of AT-cut crystal from the center of the crystal oscillator to its periphery as a result of touching it by means of the "Probe Method". The AT-cut crystal oscillator for electronic communications instruments cannot be used for watches because of the fact that the crystal oscillator itself is too large and the supporting structure is not strong enough to protect against impact damage. In addition, if a conventional AT-cut crystal oscillator of this type is made smaller, it results in a deterioration of the characteristics of the AT-cut crystal oscillator and in particular, the temperature characteristic, the aging effect and the crystal impedance.